1. Field of the Invention
The present invention relates to a seat back framework of a vehicle seat so designed to protect a body of seat occupant on the seat against damages in the case of a rear-end collision. In particular, the invention is directed to a simplified seat back framework of vehicle seat for preventing a seat occupant against upward dislocation on and along the seat back in order to protect the neck or cervical vertebrae of the seat occupant against damage in the case of rear-end collision.
2. Description of Prior Art
Hitherto, there have been known various kinds of vehicle seats designed to protect a body portion of seat occupant on the seat against damage in the case of a rear-end collision. In particular, to protect a neck (or cervical vertebrae) of seat occupant, various kinds of arrangements are known and incorporated in the vehicle seats. For example, as disclosed from the U.S. Pat. No. 7,758,114, an active headrest arrangement is provided in a seat back of vehicle seat, which comprises: a pressure receiving member movably provided in a seat back frame at a point where a seat occupant's back portion is to be positioned; and a movable headrest operatively connected with the pressure receiving member. According thereto, when a rear-end collision occurs, a backward load is applied from the seat occupant, under inertia, to the pressure receiving member which is in turn moved in the backward direction. At that moment, the seat occupant's back portion rested on that pressure receiving member well as the seat occupant's head are simultaneously displaced in the likewise backward direction. Responsive to such backward displacement of pressure receiving member, the headrest is automatically moved in forward direction, so that the seat occupant's head is quickly received by the headrest and thus protected against damage, which in effect protects a neck of the seat occupant against damage as well.
On the other hand, known is a protection arrangement in seat back, which does not require the foregoing movement of headrest, but simply requires a deformation or disconnection of constituent members in the seat back frame to bring the seat occupant's head to a stationary headrest fixed and unmoved on the seat back frame, so that the head (and seat occupant's neck as well) is protected against damage in the case of rear-end collision. For example, as disclosed from the Japanese Laid-Open Patent Publication No. 2009-107571 or JP 2009-107571 A, a spring support assembly of substantially flat configuration is disposed in a seat back frame, such that two lateral ends of the spring support assembly are releasably attached to the respective two lateral frame members of the seat back frame by means of releasable connecting members. According thereto, normally, the seat occupant's back portion is resiliently supported by that spring support assembly, but, when a rear-end collision occurs, upon receiving an excessive degree of backward load from the seat occupant, the connecting members are forcibly deformed and disconnected from the lateral frame members of seat back frame. Due to such disconnection of connecting members, the spring support assembly as well as the seat occupant's body are further displaced backwardly beyond a normal backward support limit, whereby the seat occupant's head is quickly received by the stationary headrest and protected against damage, which therefore protects his or her neck as well.
However, the above-described prior art seat back structures have encountered the following defective aspects:                (i) In assembly of the seat back, a great number of parts and mechanisms are required, which results in a complicated and time-consuming construction of the seat back and a highly increased costs involved.        (ii) All the arrangements stated above do not take into account the fact that the seat back itself is normally in a backwardly inclined state, which means that the seat back provides a condition for allowing the back portion of seat occupant to be easily slipped and slid on and along the inclined surface of seat back. As a consequence thereof upon occurrence of rear-end collision, the seat occupant's upper body portion is sunk into the backwardly inclined surface of seat back, but quickly slipped upwards at the same time on and along such inclined surface of seat back, thus increasing the possibility that the seat occupant's head will be dislocated upwards from a predetermined support point in the headrest, and therefore the seat occupant's head will not be properly received by the headrest, which will result in his or her neck (i.e. cervical vertebrae) being damaged.        